Helical corrugated metal pipe is formed from an elongated strip of sheet metal. Typical pipe forming machines corrugate the sheet metal by passing it through a series of rollers; the machines then form the pipe by forcing the corrugated sheet metal through a three roll bending assembly to produce a spiral. A pipe seam is formed from interlocking flanges produced on outer sheet metal edges during sheet metal corrugation. The seam is sealed as a first interlocking corrugated sheet metal edge spirals around to meet a second interlocking corrugated sheet metal edge. The two edges meet and are joined between a set of compressive rollers.
The three roll bending assembly is a standard in the corrugated pipe forming industry, its operation is well understood. The assembly consists of a guide roll, an anvil roll, and a pressure roll. As corrugated sheet metal is forced between the rolls, the anvil roll acts as a fulcrum causing the strip to bend. The diameter is formed pipe is nominally a function of the helix angle (the angle at which the corrugated sheet metal strip is inserted between the three rolls). The inside diameter of the pipe formed is also affected by the actual width of sheet metal strip used to form the pipe, the corrugation depth, the strip thickness, the width of the corrugation containing the lockseam, and the position of the lock up rolls relative to the other bending rolls.
It is difficult to maintain an accurate pipe diameter for a long run of pipe. Industry standards dictate that inside pipe diameter cannot vary more than 1/2-inch under nominal value for pipes up to 48-inches in diameter and not more than 1% under nominal values for pipes greater than 48-inches in diameter (in no event may adjacent ends differ by more than 1 inch). Unfortunately, the three roll process of forming helical corrugated pipe is subject to a phenomenon known as "funneling". A pipe initially produced in a desired diameter can begin to "creep" to a progressively larger or smaller diameter. The progressively increasing or decreasing diameter pipe is rejected because it has a diameter that goes beyond its tolerance, the pipe somewhat resembling a long funnel.
Also of critical importance regarding pipe diameter accuracy is that of raw material cost. Currently, sheet metal is purchased for approximately $600 per ton. If the pipe forming machine processes 5,000 tons of sheet metal per year at $600 per ton, a 21/2% saving amounts to $75,000 per year. Accurate pipe diameter is much sought after, as indicated by the prior art. A common method of controlling pipe diameter is a guide chain wrapped about the circumference of the formed pipe. The guide chain forces the pipe within a preset diameter as determined by the length of the guide chain. Illustrative of guide chain diameter control is the Ohnstad invention, U.S. Pat. No. 3,092,056 (issued June 4, 1963); the Wiley invention, U.S. Pat. No. 3,256,724 (issued June 21, 1966); and the Hutton invention, U.S. Pat. No. 3,499,307 (issued Mar. 10, 1970). A problem with this method of controlling pipe diameter is that changing to different size diameters is troublesome. Other problems include: the chain can mark, score, or deform the pipe being formed, the chain pushes the formed pipe back toward the forming rollers, and shifting of the chain about the pipe causes the diameter of the formed pipe to vary from the desired diameter.
Other methods of controlling pipe diameter involve controlling the linear speed at which the pipe is formed (Crawford, U.S. Pat. No. 3,309,003 issued Mar. 14, 1967), controlling angular shear forces within the bending rolls (Krakow, U.S. Pat. No. 3,991,597 issued Nov. 16, 1976), and adjusting the lock up rolls radially relative to the pipe axis (Davis, U.S. Pat. No. 3,940,962, issued Mar. 2, 1976).
Measuring diameter is essential no matter how the diameter is controlled; all the diameter control systems are subject to pipe funneling. The two most common methods of measuring pipe diameter are the use of a chain (the Fay invention, U.S. Pat. No. 3,393,546 issued July 23, 1968; and the Hutton invention) and using a ruler or tape measure (Davis).
The chain method is subject to the problems discussed above for chain diameter control. Due to the twisting of the links, the corrugations of the sheet metal pipe, the deformation of the pipe, and the stretching of the chain, accuracy is far from acceptable.
More commonly, a ruler or tape measure is used to measure pipe diameter. As recently as 1976 (Davis), the ruler or tape measure was advocated as a state of the art in pipe diameter measuring. It is difficult to accurately measure a corrugated helical pipe by placing a band around it, whether the band be a chain or a tape measure. The corrugations run helically around the pipe and at best, a rough measurement of the outer diameter can be obtained. Orienting the tape measure 90.degree. to the pipe axis is very difficult. There are no axial references on the pipe and the corrugations tend to create optical confusion when attempting such alignment. The most significant disadvantage of using a tape measure or ruler is the need to stop the pipe forming process to make the measurement accurately. Continual stopping wastes time, reduces productivity, and exacerbates pipe diameter error.
In addition to pipe diameter measurement, formed pipe length is subject to certain industry standards. These standards allow a maximum of 1% cumulative length variation per shipment between length of pipe formed and length of pipe desired. Assuming $600 per ton for sheet metal and 5,000 tons of sheet metal processed anually, a 1% savings in length amounts to $30,000 per year. The combined length and diameter savings resulting from accurate measurement of diameter and length can approximate $105,000 per year.
As can be seen from the foregoing discussion, productivity and efficiency in the formation of corrugated metal pipe are hampered by current methods to meet the pipe forming industry's length and diameter requirements. Continual starting and stopping of the pipe forming machine to make measurements of formed pipe length and diameter is undesirable due to decreased efficiency and due to increased wear and tear on the pipe forming machine. While all the foregoing problems have plagued the industry for some time, the helical corrugated pipe forming art is still advocating the old methods of rulers, tape measures, and chains.